Ferrous metal products are commonly rendered corrosion resistant by the application of electroplated coatings of non-ferrous metals such as zinc, nickel, copper, cadmium and chromium. Electroplate is also employed to provide a decorative finish on many types of metal products. Zinc is the metal which is predominantly employed for electroplating.
Presently, more than 40% of the plating shops in the United States electroplate zinc from a variety of plating baths. These include highly alkaline plating solutions, which often contain cyanide as a complexing agent, and acidic baths containing chloride or sulfate salts and buffers for pH control. Other less commonly-used acidic baths use fluoroborate and pyrophosphate salts. Proprietary organic additives are often used to produce a bright finish.
Each year, zinc plating operations in the United States generate five billion gallons of wastewater contaminated with zinc, which must be removed prior to release of the wastewater into the environment. The majority of zinc platers currently employ conventional wastewater treatment techniques for the removal of zinc. Typically, the pH of the wastewater is adjusted to about 8-11 to precipitate the metallic cations as the corresponding hydroxides, which are gravity concentrated and dewatered to yield a toxic heavy metal sludge.
Even when highly concentrated, this sludge is not reusable in plating baths, since it contains substantial amounts of iron and water hardness factors (calcium and magnesium salts). Therefore, the sludge must be containerized and shipped to environmentally-secure landfill sites. The total cost of such "safe" permanent disposal is very high, and can equal or exceed the value of the chemicals used in the plating process. The number of these hazardous waste disposal sites is declining. Many of the sites will require costly remedial action under state and federal laws which hold the waste generator to be ultimately responsible for clean-up costs. The proposed USEPA Disposal Elimination Schedule will force platers to employ metal recovery methods by banning land disposal of such hazardous electroplating wastes by 1988. See Federal Register (May 13, 1985).
In order to avoid the formation of these intractable chemically-complex metal waste sludges, processes have been developed based on the selective precipitation of metal values from aqueous solutions of dissolved metals. For example, U.S. Pat. No. 3,800,024 discloses a process for the sequential removal of iron, chromium and nickel from spent acidic steel pickling solutions by raising the pH of the solution in two steps. R. F. Pagel, (U.S. Pat. No. 4,025,430), discloses the sequential treatment of an acidic effluent with a controlled amount of lime, followed by precipitation of the metal ions remaining in the overflow with sodium silicate. However, such processes do not address the problems associated with recovering the metal values from alkaline plating wastewaters and waste metal sludges containing mixtures of precipitated metal hydroxides. Furthermore, the presently-available technologies such as ion exchange, reverse osmosis, evaporation and electrolysis cannot economically separate and recover the metal values from these complex wastewaters and sludges.
Therefore, a need exists for a method to remove the metal values from metal-containing wastewaters and sludges to afford effluents which can be safely discharged into the environment. A further need exists for a method to separate non-ferrous metals from the ferrous metal and water hardness factor impurities present in these aqueous metal wastes to yield a reusuable non-ferrous metal-containing product.